This application is generally related to co-pending U.S. application Ser. Nos. 625,792 and 598,434, and to U.S. Pat. Nos. 4,521,680 and 4,585,933.
The present invention relates to an optical head for focusing a light beam for reading information on an information recording medium such as an optical disk, and more particularly, to an improvement in photo detector for an optical head for detecting a whether the light beam is focused.
Various optical information recording/reproduction systems have been recently devised which optically write and read information on and from an information recording medium (to be referred to as an optical disk hereinafter). For example, information recording/reproduction systems are known in which information is only read from a recording medium such as a CD (compact disk) type of DAD or a video disk, an image file, a still image file, a COM (computer output memory) or the like. In such systems, information is initially recorded on a medium by establishing a state change such as forming a hole (pit) in a recording layer by a focused light beam. In other information recording/reproduction systems, information is optically written on or read from an information recording medium for recording/reproduction or from an erasable information recording medium (to be referred to as an optical disk hereinafter). In these systems, in both the write and read modes, a light beam must always be focused on the optical disk. In other words, the beam waist of the light beam must be coincident with the optical disk surface, and a minimum-sized beam spot must be formed on the optical disk. In view of this requirement, the optical head frequently has a focus detecting system for detecting the state of focus of the light beam. Various such focus detecting systems have been proposed. For example, U.S. Ser. No. 399,873, filed on July 19, 1982, now U.S. Pat. No. 4,521,680, and corresponding EPC application No. 82106508.3, filed on July 19, 1982 disclose a focus detecting system adopting the so-called knife edge method.
In this focus detecting system adopting the knife edge method, as shown in FIG. 1, a laser beam is reflected from a recording layer or light reflecting layer 10 on which information has to be or is already been recorded. An objective lens 2, a knife edge 4, a convergent lens 6 and a photo detecting unit 8 are arranged on an optical path O--O of the reflected light from the recording layer 10. The knife edge 4 serves to allow passage of only that laser beam component which is spaced apart from the optical axis 0--0. The photo detecting unit 8 detects a laser beam which is converged by the convergent lens 6. The photo detecting unit 8 has two, first and second, photo sensing regions or photo sensitive regions 8-1 and 8-2, and is located at the back focal point of the convergent lens 6. In such a focus detecting system, the defocusing state is not detected by a change in the beam spot size on the photo detecting unit 8 but is detected by a deviation in the beam spot position in a direction 9. In the just in-focusing state, the beam spot is formed at the boundary between the two photo sensitive regions 8-1 and 8-2 of the photo detecting unit 8. A differential signal of the two photo signals from these regions 8-1 and 8-2 is kept substantially zero. In contrast to this, when the objective lens 2 draws too near to the recording layer 10 or is spaced too far apart therefrom, resulting in the defocusing state, the differential signal of the signals from the first and second photo sensitive regions 8-1 and 8-2 changes in the positive or negative direction. The level of this differential signal depends on the distance between the objective lens 2 and the recording layer 10, as shown in FIG. 2. In FIG. 2, the distance .alpha. between the objective lens 2 and the recording layer 10 in the just in-focusing state is set to be 0, and the distance has a positive value when the two members are spaced too far apart from each other and has a negative value when the two parts draw too close to each other.
The focus detecting system as described above has various problems as will be described below:
(1) In order to allow information recording at a high density, and fast and correct information reproduction, tracking guides are formed in an optical disk in the form of grooves or projections. When information is recorded in the form of pit arrays, pit arrays are recorded on the optical disk. When such tracking guides or pit arrays are irradiated with a laser beam, the irradiated light is diffracted by the tracking guides or pit arrays. Thus, a diffraction pattern of an irregular shape is formed on photo sensitive regions of a photo detecting unit which receives light reflected by the optical disk. This diffraction pattern is formed as a dark portion in the beam spot pattern which is formed on the photo sensitive regions of the photo detecting unit. For this reason, in the optical head which detects the state of focus in accordance with a difference between signals obtained from photo sensitive regions, the defocusing state can be erroneously detected to be the just in-focusing state or vice versa due to the diffraction pattern. A similar problem may also occur when there is a defect in the optical system of the optical head or when the laser beam is diffracted by dust or the like.
(2) In an optical disk, in general, the recording layer 10 is formed on a transparent substrate, and a laser beam converged by the objective lens 2 is projected onto the recording layer 10 through the substrate. When the optical disk deforms or becomes bent and the substrate is accordingly deformed, an aberration such as a coma is produced. When the aberration such as a coma is produced, the image pattern including the coma is formed on the photo sensitive regions 8-1 and 8-2 in the just in-focusing state. Even if the objective lens system has only a small coma aberration, if the defective pattern due to such a coma aberration is formed on the photo sensitive regions 8-1 and 8-2, the defocusing state can be erroneously detected to be the just in-focusing state or vice versa.
(3) Near the focal point of the convergent lens 6, the beam spot has a specific size due to the influence of light diffraction (wave characteristics of light). For this reason, when the photo detecting unit is arranged at the focal point of the convergent lens 6 in the just in-focusing state, the defocusing detection sensitivity near the focal point is lower than the theoretical value.
(4) The beam spot size at the focal point of the convergent lens 6 is very small. In the case of an optical system in which the photo detecting unit is arranged at the focal point of the convergent lens, a slight change in the position of the photo detecting unit due to a change in temperature results in an erroneous detection of the just in-focusing state as the defocusing state.
(5) When there is a large aberration, the positions of the Gauss image plane and the least circle of confusion deviate from each other. Therefore, the focus detection characteristics near the just in-focusing position are degraded, and the detection sensitivity is degraded.